[feature] Stub out trends + suggestions (always return empty array) (#4435)

# Description

> If this is a code change, please include a summary of what you've coded, and link to the issue(s) it closes/implements.
>
> If this is a documentation change, please briefly describe what you've changed and why.

This pull request stubs out the trends and suggestions APIs, just returning empty arrays for all four of the added endpoints. This is to help clients show fewer errors. It does *not* signal any intention to actually implement these endpoints properly, though you never know.

closes https://codeberg.org/superseriousbusiness/gotosocial/issues/4385

## Checklist

Please put an x inside each checkbox to indicate that you've read and followed it: `[ ]` -> `[x]`

If this is a documentation change, only the first checkbox must be filled (you can delete the others if you want).

- [x] I/we have read the [GoToSocial contribution guidelines](https://codeberg.org/superseriousbusiness/gotosocial/src/branch/main/CONTRIBUTING.md).
- [x] I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
- [x] I/we have not leveraged AI to create the proposed changes.
- [x] I/we have performed a self-review of added code.
- [x] I/we have written code that is legible and maintainable by others.
- [x] I/we have commented the added code, particularly in hard-to-understand areas.
- [x] I/we have made any necessary changes to documentation.
- [ ] I/we have added tests that cover new code.
- [x] I/we have run tests and they pass locally with the changes.
- [x] I/we have run `go fmt ./...` and `golangci-lint run`.

Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4435
Co-authored-by: tobi <tobi.smethurst@protonmail.com>
Co-committed-by: tobi <tobi.smethurst@protonmail.com>

docs/api/swagger.yaml

@@ -12115,6 +12115,32 @@ paths:
             summary: Initiate a websocket connection for live streaming of statuses and notifications.
             tags:
                 - streaming
+    /api/v1/suggestions:
+        get:
+            description: 'THIS ENDPOINT IS CURRENTLY NOT FULLY IMPLEMENTED: it will always return an empty array.'
+            operationId: getSuggestions
+            produces:
+                - application/json
+            responses:
+                "200":
+                    description: ""
+                    schema:
+                        items:
+                            type: object
+                        maxItems: 0
+                        type: array
+                "401":
+                    description: unauthorized
+                "403":
+                    description: forbidden
+                "406":
+                    description: not acceptable
+            security:
+                - OAuth2 Bearer:
+                    - read
+            summary: Accounts that are promoted by staff, or that the user has had past positive interactions with, but is not yet following.
+            tags:
+                - suggestions
     /api/v1/tags/{tag_name}:
         get:
             description: If the tag does not exist, this method will not create it in the database.
@@ -12567,6 +12593,63 @@ paths:
             summary: Invalidate the target token, removing it from the database and making it unusable.
             tags:
                 - tokens
+    /api/v1/trends/links:
+        get:
+            description: 'THIS ENDPOINT IS CURRENTLY NOT FULLY IMPLEMENTED: it will always return an empty array.'
+            operationId: getTrendingLinks
+            produces:
+                - application/json
+            responses:
+                "200":
+                    description: ""
+                    schema:
+                        items:
+                            type: object
+                        maxItems: 0
+                        type: array
+                "406":
+                    description: not acceptable
+            summary: Links that have been shared more than others.
+            tags:
+                - trends
+    /api/v1/trends/statuses:
+        get:
+            description: 'THIS ENDPOINT IS CURRENTLY NOT FULLY IMPLEMENTED: it will always return an empty array.'
+            operationId: getTrendingStatuses
+            produces:
+                - application/json
+            responses:
+                "200":
+                    description: ""
+                    schema:
+                        items:
+                            type: object
+                        maxItems: 0
+                        type: array
+                "406":
+                    description: not acceptable
+            summary: Statuses that have been interacted with more than others.
+            tags:
+                - trends
+    /api/v1/trends/tags:
+        get:
+            description: 'THIS ENDPOINT IS CURRENTLY NOT FULLY IMPLEMENTED: it will always return an empty array.'
+            operationId: getTrendingTags
+            produces:
+                - application/json
+            responses:
+                "200":
+                    description: ""
+                    schema:
+                        items:
+                            type: object
+                        maxItems: 0
+                        type: array
+                "406":
+                    description: not acceptable
+            summary: View hashtags that are currently being used more frequently than usual.
+            tags:
+                - trends
     /api/v1/user:
         get:
             operationId: getUser